This invention relates to filters for electromagnetic energy, and more particularly to such filters which use plasmas.
Plasma is said to be the most common state of matter in the universe. In essence, a plasma is a state of matter in which the electrons of an atom are free from the remainder of the atom, whereby the remainder of the atom is electrically charged or ionized. Plasma has been of interest in the field of communications because of the presence of the ionosphere, which is a plasma lying above the earth""s surface. It has long been known that the ionosphere would reflect electromagnetic radiation (also known generally as xe2x80x9cradio-frequencyxe2x80x9d signals or radiation), so long as the frequency of the electromagnetic radiation was below a calculable frequency. The frequency at which the ionosphere reflects is known to vary as a function of at least the charged particle density of the ionosphere.
There is a body of knowledge about plasmas, set forth in texts such as Fields and Waves in Communication Electronics by Ramo, Whinnery, and Van Duzer, published 1994 by Wiley and Introduction to Plasma Physics and Controlled Fusion, by Chen, published 1994 by Plenum Press. In addition, the Internet is a source of information in regard to plasmas. A fundamental time-scale in plasma physics is the xe2x80x9cplasma frequency.xe2x80x9d Such a plasma frequency exists for all conductors, where the term xe2x80x9cconductorxe2x80x9d refers to matter containing free electrons.
The plasma frequency op is given by                               O          p          2                =                              ne            2                                              ϵ              0                        ⁢            m                                      1      
where:
n is the charged particle density (charged particle/volume);
e is the charge of the ionized particle (ion or electron);
m is the mass of the charged particle; and
o is permittivity of free space.
It will be clear that the plasma frequency is different as between the electron and the ion species, because of their different masses. The plasma frequency which is commonly referred to in the literature is the electron plasma frequency, which is much higher than the ion plasma frequency. The plasma frequency may be viewed simplistically as being the frequency in a conductor at which the electrons collectively oscillate relative to their ion cores. The plasma frequency may be conveniently measured in a manner analogous to the ionosphere propagation model, namely by applying an electromagnetic signal to the plasma, and determining the frequency at which reflection changes to propagation.
A frequency-sensitive apparatus according to an aspect of the invention comprises a first path for the flow of electromagnetic energy, and a first plasma lying in the first path. The first plasma has a first plasma frequency. A second plasma lies in the first path. The second plasma has a second plasma frequency, different from the first plasma frequency. In a particular embodiment of the apparatus according to this aspect of the invention, an electromagnetic energy port is coupled to the first path at a location lying between the first and second plasmas. In another embodiment, a first circulator is provided. The first circulator defines first, second, and third ports, and first, second, third paths for the flow of circulator electromagnetic energy from the first port to the second port, from the second port to the third port, and from the third port to the first port, respectively. The first path for the flow of electromagnetic energy of the first circulator lies in the first path for the flow of electromagnetic energy, at a location along the first path for the flow of electromagnetic energy which lies between the first and second plasmas. In another embodiment of this aspect of the invention, a second circulator is provided, with the second circulator defining fourth, fifth, and sixth ports, and fourth, fifth, and sixth paths for the flow of second circulator electromagnetic energy from the fourth port to the fifth port, from the fifth port to the sixth port, and from the sixth port to the fourth port, respectively, of the second circulator. The fourth port is coupled to the first path for the flow of electromagnetic energy at a location, relative to the first plasma, which is remote from the first circulator. In particular embodiments of this aspect of the invention, a first voltage source is coupled to the first plasma for maintaining the first plasma at the first plasma frequency, and a second voltage source is coupled to the second plasma for maintaining the second plasma at the second plasma frequency. In one application of an apparatus according to this first aspect of the invention, a source of electromagnetic energy is coupled to the first path at a location adjacent the first plasma, for transmitting electromagnetic waves along the path toward the first plasma and the second plasma. In a particular application, the electromagnetic waves transmitted by the source of electromagnetic energy include a component at a frequency lower than the plasma frequency of the first plasma, whereby the component of the electromagnetic waves transmitted by the source of electromagnetic energy tends to be reflected by the first plasma. In another particular application of the apparatus according to this aspect of the invention, the electromagnetic waves transmitted by the source of electromagnetic energy include a component at a frequency lying between the plasma frequency of the first plasma and the plasma frequency of the second plasma, whereby the component of the electromagnetic waves transmitted by the source of electromagnetic energy tends to be passed by the first plasma and reflected by the second plasma. In this particular application, it may be advantageous if the apparatus further comprises electromagnetic signal coupling means coupled to the first path at a location lying between the first and second plasmas, for extracting the component of the electromagnetic energy at a frequency lying between the plasma frequency of the first plasma and the plasma frequency of the second plasma. In one version, the electromagnetic signal coupling means is directional. It may be a directional coupler. In yet another application of the apparatus according to this aspect of the invention, the electromagnetic waves transmitted by the source of electromagnetic energy include a component at a frequency lying above the plasma frequencies of the first plasma and the second plasma, whereby the component of the electromagnetic waves transmitted by the source of electromagnetic energy tends to be passed by the first and second plasmas. In such a version, the apparatus may advantageously include utilization means coupled to the first path at a location relative to the second plasma which is remote from the first plasma. In an application of the apparatus according to this aspect of the invention, where the electromagnetic waves transmitted by the source of electromagnetic energy include components having frequencies lying above the plasma frequency of the first plasma and other components having frequencies lying above the plasma frequency of the second plasma the apparatus according to this aspect of the invention may advantageously include electromagnetic signal coupling means, which may be directional, coupled to the first path at a location lying between the first and second plasmas, for extracting the component of the electromagnetic energy at a frequency lying between the plasma frequency of the first plasma and the plasma frequency of the second plasma. It may further advantageously include signal utilization means coupled to the first path at a location adjacent the second plasma and remote from the first plasma, for utilizing the component of the electromagnetic energy at a frequency lying above the plasma frequency of the second plasma. If the electromagnetic signal coupling means comprises a directional coupler, the coupler may define a path between first and second ports, and a third port, with the path between first and second ports lying in the first path at the location lying between the first and second plasmas, for coupling to the third port of the directional coupler the electromagnetic energy at a frequency lying between the plasma frequency of the first plasma and the plasma frequency of the second plasma which reflect from the second plasma. For those applications of the apparatus according to this aspect of the invention, where the source of electromagnetic energy further generates waves with components at frequencies below the plasma frequency of the first plasma, the apparatus may include coupling means lying between the source of electromagnetic energy and the first plasma, for extracting that component of the electromagnetic energy with components at frequencies below the plasma frequency of the first plasma which are reflected by the first plasma.
According to another aspect of the invention, a frequency-sensitive apparatus comprises a first plasma defining first and second ports, and a path for the flow of electromagnetic energy between the first and second ports of the first plasma at frequencies lying above a plasma frequency of the first plasma. A second plasma defines first and second ports, and a path for the flow of electromagnetic energy between the first and second ports of the second plasma at frequencies lying above a plasma frequency of the second plasma. A source of electromagnetic energy generates electromagnetic waves including at least one component. The component has a frequency which is one of (a) below the first plasma frequency, (b) between the first and second plasma frequencies, and (c) above the second plasma frequency. A first directional coupling means is provided. The first directional coupling means includes first, second, and third ports, for coupling signal in a directional manner, namely from the first port exclusively to the second port, from the second port exclusively to the third port, and from the third port exclusively to the first port. The second port of the first directional coupling means is coupled to the first port of the first plasma, for coupling to the third port of the first directional coupling means at least that component of the electromagnetic energy which reflects from the first plasma. The apparatus according to this aspect of the invention also includes second directional coupling means including first, second, and third ports, for coupling signal in a directional manner from the first port exclusively to the second port, from the second port exclusively to the third port, and from the third port exclusively to the first port. The first port of the second directional coupling means is coupled to the second port of the first plasma, and the second port of the second directional coupling means is coupled to the first port of the second plasma, for coupling to the third port of the second directional coupling means at least that component of the electromagnetic waves which reflects from the second plasma. In a particular version of this aspect of the invention, the apparatus further comprises utilization means coupled to the second port of the second plasma, for utilizing those components of the electromagnetic wave passing through the second plasma.
An apparatus according to another aspect of the invention includes a first plasma, and a first radio-frequency electromagnetic coupling means coupled to the first plasma, for defining first and second radio-frequency electromagnetic ports of the first plasma. This apparatus also includes a second plasma, and second radio-frequency electromagnetic coupling means coupled to the second plasma, for defining first and second radio-frequency electromagnetic ports of the second plasma. The apparatus according to this other aspect of the invention also includes a radio-frequency electromagnetic path extending from the second port of the first plasma to the first port of the second plasma, for coupling radio frequencies electromagnetic waves between the second port of the first plasma and the first port of the second plasma. In a version of this other aspect of the invention, the radio-frequency electromagnetic path comprises a directional coupler. In this version, the directional coupler may comprise first, second, and third ports, and it may couple (a) signals applied to the first port of the directional coupler to the second port of the directional coupler, (b) signals applied to the second port of the directional coupler to the third port of the directional coupler, and (c) signals applied to the third port of the directional coupler to the first port of the directional coupler. The first port of the directional coupler is coupled to the second port of the first plasma, and the second port of the directional coupler is coupled to the first port of the second plasma, for coupling to the third port of the directional coupler electromagnetic signals reflected from the first port of the second plasma. A second directional coupler may be used. The second directional coupler may define first, second, and third ports, for coupling (a) signals applied to the first port of the second directional coupler to the second port of the second directional coupler, (b) signals applied to the second port of the second directional coupler to the third port of the second directional coupler, and (c) signals applied to the third port of the second directional coupler to the first port of the second directional coupler. The second port of the second directional coupler is coupled to the first port of the first plasma, for coupling to the third port of the second directional coupler signals reflected from the first port of the first plasma.